Preheat the oven to 300°F.
Put some flour into any bowl (I used a soup bowl) to dredge the veal cubes with flour. You can season the flour if you like with a little salt and pepper or your favorite dried seasonings. Be sure the veal is dry before dredging. Heat your sauté pan over medium-high heat; add 2 tablespoons of butter and 2 tablespoons of oil to the pan. When the oil is hot but not smoking, shake off any excess flour from the veal pieces and add them to the sauté pan. Most likely you'll sauté the veal in batches unless you have one giant sauté pan.All we are trying to do is brown the veal on all sides. The cooking will be done in the liquids. The browning will add color and add flavor to the pan that will result in a very tasty stew. When one batch of veal is browned, remove it to a plate and start the next batch. When all is meat is browned and reserved, add another tablespoon of butter and oil to the pan and add the leeks, pearl onions and garlic. Sauté the vegetables for about 5 to 7 minutes until the leeks become translucent & slightly brown. Add the port wine and deglaze the pan by scraping from the bottom any meat or leeks that may be stuck to the pan. Continuing cooking until most of the port is cooked off, then add the meat back to the pan.
At this point you want to add the cut up tomatoes with juice along with the cup of demi glace. Add the sage, parsley and season with salt and pepper.
Cover and place the pan in the preheated oven and cook for about 1 hour.
While the veal stew is cooking, it's time to sauté the mushrooms in a smaller sauté pan. You want to sauté the mushrooms separately so they release their liquid before adding to the stew.